if you run on callmanager :
did you check that your TFTP server is running and reachable from VLAN where the phone are located.
i would first restart the TFTP service
Check in application/Cisco callmanager Serviceability. then tools/control center.
select your TFTP server and check the service is rnning. try to restart aslo, then restart one of the faulty phone and check if Ring tone are back.
for Callmanager 4.x, ringtone are located on the TFTP server in C:\Program Files\Cisco\TFTPPath
there is a file call ringList.xml that contain the list of ringtone provide to user and accessible only on the first DN, other DN can have only few ring tone.